Overall, both hydrology and subsystem type had significant effects on both the amount and relative abundance of nutrients and ions in Sycamore Creek ( Table III). Nitrate dominated in the active channel, and ammonium dominated in the riparian zone. Subsystems also differed in the relative abundance of nitrate and ammonium ( Table IV).
Similar to the riparian pattern, nutrients in the active channel increased after floods ( Table II), but differences between riparian zone and active channel persisted for nutrients and were accentuated for conductivity ( Table II). Conductivity was significantly higher in the riparian zone. During baseflow, water in the riparian zone was characterized by lower DIN, SRP, and dissolved oxygen concentrations relative to the active channel ( Table II). Sharp differences in subsurface water chemistry occurred across the interface between the active channel and the riparian zone at all times. Riparian zone chemistry, however, did not differ from water sampled from wells located in the stream bank. Water chemistry in the riparian zone differed significantly from both subsurface and surface water in the active channel ( Table III). This method-illustrated in the following example-requires defining duplicate components for each gas: H2S(aq) and HS-, CO2(aq) and HCO3-, NH3(aq) and NH4+, etc.
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The user then prepares the input matrix ( File : Access Library … ), selects H 2S(aq) as the hydrogen sulfide component, enter the pH-independent aqueous dihydrogen sulfide concentration, and select free mode. This method is not recommended for novice ChemEQL users.Īlternatively-using results from the preceding example-the user can calculate the pH-independent aqueous dihydrogen sulfide concentration c H 2S for the chosen gaseous dihydrogen sulfide partial pressure p H 2S. The user-after calculating the partial pressure-dependent Henry’s Law coefficient k H, H 2 S ‡ as illustrated in the preceding example-can edit the database, replacing the equilibrium constant each time a simulation is performed. The following example will show aqueous dihydrogen sulfide H 2S(aq) is independent of solution pH and depends solely on the dihydrogen sulfide H 2S(g) partial pressure ( Example 5.D.4).ĬhemEQL allows two methods for replicating the fixed fugacity method. The ChemEQL model does not implement the fixed fugacity ( Delany and Wolery, 1984) and requires another approach to simulating water in equilibrium with a limitless gas-phase reservoir. ChemEQL requires the user to account for the detailed chemistry of gases that undergo hydrolysis when dissolve in water (e.g., carbon dioxide CO 2, dihydrogen sulfide H 2S and ammonium NH 3) ( Example 5.D.3). There is no question the fixed fugacity method implemented by most numerical water chemistry applications demands less from the user, the ChemEQL approach offers a decided advantage for novice users. The second is the approach required by the ChemEQL model. The first is a concrete example of the fixed fugacity method ( Delany and Wolery, 1984).
This section covers two examples designed to illustrate how a user simulates natural water with dissolved gases. Dissolved dioxygen O 2(aq) levels are often close to saturation, the exception being carbon-rich sediments and aquifers where bacteria consume virtually all dissolved oxygen. For many important gases found in the above ground atmosphere the extremely long groundwater residence time is sufficient for gases to saturate the water. The reader might assume groundwater simulations could disregard dissolved gases, yet even in where direct contact between water and a gas phase is not apparent dissolved gases cannot be neglected. Surface waters and soil pore water are two systems where including dissolved gases in water chemistry simulations is absolutely essential. Natural water chemistry requires, in many instances, an account of dissolved gaseous. William Bleam, in Soil and Environmental Chemistry (Second Edition), 2017 5.D.2 Aqueous Solubility of Gases
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